The present invention relates to an apparatus for applying a pressure on the surface of progressively moving workpieces such as wood or chipboard panels. A known type of apparatus has at least one revolving endless pressure application belt which is adapted to be pressed against the workpiece by means of a pressure medium. The pressure medium is located with a pressure chamber on the internal surface of the belt, the chamber being fitted with sealing strips.
In such known devices, there is provided a single pressure chamber located above or below the belt in dependence upon the location of the belt with respect to the workpiece. Such chamber extends over substantially the entire width of the progressively moving workpiece. Such large pressure chambers, which are impacted by a pressure medium such as compressed air, have inherent adverse features. The sealing strips which extend in the direction of movement of the pressure application belt, on the open side of the chamber are subjected to considerably higher stresses than the sealing members at right angles thereto. This leads to different wear characteristics, premature tearing of the strips and loss of air-tightness. The necessarily long sealing strips cannot be made in one integral piece. There are therefore one or more abutment points which also cause air leakage and pressure losses. Considerable forces act on those sections of the sealing strip which extend in the direction of travel of the belt and so these strips must be supported. This, however, necessitates considerable structural expenditure. If there is a sudden considerable drop in pressure in the chamber, the workpiece presses the belt against the internal surfaces of the pressure chambers. Considerable wear occurs if such internal surfaces are not coated with a special low-friction material. These preparations required to give emergency running properties are also expensive, and are difficult to produce.
The present invention seeks to develop such an apparatus in a manner such that the sealing strips do not have abutment points or have joints, do not require costly or complicated supports, and which exhibit good emergency running properties. The present invention seeks to provide an apparatus in which sealing strips are readily interchangeable. Moreover, the present invention seeks to provide an arrangement in which the pressure application is adaptable, in a simple manner, to differing workpiece widths.
In accordance with the present invention, there is provided an apparatus for applying a surface pressure to progressive moving workpieces comprising at least one rotating pressure belt having a workpiece carrying surface and an opposed surface and means for applying pressure to said belt to cause said belt to apply pressure to said workpiece, said pressure application means comprising a sealing strip in sealing abutment with said opposed surface of said belt, said sealing strip together with said belt forming part of a pressure chamber into which a pressure medium is induced the improvement comprising the provision of a plurality of pressure chambers over the length and width of said belt in its workpiece carrying region. Reduction of the size of the pressure chambers inherent in such an arrangement improves the life and the effectiveness of the sealing strips secured to the chambers.
Preferably, said pressure chambers are cylindrical and have a cross-section which is selected from the group consisting of triangular, oval, circular and elongate rectangular with radiused ends. By means of a plurality of such small pressure chambers the sealing problem is solved in a particularly favourable manner. On the one hand such pressure chambers, especially those having a circular cross-section, that is to say, those which are right cylinders are very easy to produce. The circular sealing strips may be produced in a single piece without joints. On account of such shape the sealing member or strip is well supported all around the chamber and premature wear and tear is avoided. Because of the relatively small ring surface area, the surfaces thereof may be machined very precisely. This eliminates any difficulties which may be encountered due to inaccuracies in machining, which difficulties have occurred in known sealing members.
Such small sealing strips are readily interchanged or replaced. Simply by connecting further small pressure chambers into the apparatus effective surface area of the pressing belt acting on the workpiece may be widened or lengthened as desired. Thus, a system of virtually any size can be constructed using the same components. It is further possible to combine the small pressure chambers in rows which are juxtaposed in the direction of travel of the belt and which rows can be impaced separately with a pressure medium. This permits the production of varying pressure forces over the width of the effective surface and, conversely, provides a very simple manner of varying the effective working width of the pressure application surface of the belt.
Desirably, the apparatus additionally comprises a pressure plate, said pressure plate having a recessed wall portion formed therein to define a groove, said groove receiving a frame member, said frame member guiding said sealing member, said sealing member being slidingly engaged by said pressure belt, said frame and said groove in said pressure plate together defining a gap, an elastic sealing member having a first end portion and a second end portion, said first end portion connecting said sealing member to said frame and said second end portion slidingly abutting said recessed wall portion defining said groove. The frame is preferably laterally supported, at its end facing the belt, on a plurality of fixed holding yokes projecting into the region of the groove. Such arrangement ensures that the frame cannot tilt in the groove even if the compressed air contained in the pressure chamber would tend to produce such an effect. The frame may thus be pressed constantly against the belt by the pressure medium whilst still being able to move slightly to and fro within the groove.